$$
begin{align*}
& phi(x,y) = phi left(sum_{i=1}^n x_ie_i, sum_{j=1}^n y_je_j right)
= sum_{i=1}^n sum_{j=1}^n x_i y_j phi(e_i, e_j) =
& (x_1, ldots, x_n) left( begin{array}{ccc}
phi(e_1, e_1) & cdots & phi(e_1, e_n)
vdots & ddots & vdots
phi(e_n, e_1) & cdots & phi(e_n, e_n)
end{array} right)
left( begin{array}{c}
y_1
vdots
y_n
end{array} right)
end{align*}
$$
[x = {-b pm sqrt{b^2-4ac} over 2a}.]
When (a ne 0), there are two solutions to (ax^2 + bx + c = 0) and they are [x = {-b pm sqrt{b^2-4ac} over 2a}.]
\[x = {-b \pm \sqrt{b^2-4ac} \over 2a}.\]